Mechanism And Effects Of Muscle Fiber Types On Beef Tenderness

Tenderness is one of the most important traits of beef quality. It is the key factor to determine whether the consumers will choose the beef or not. Aging can improve beef quality, especially beef tenderness. Muscle fiber type is the basic unit of muscle and it was closely related to beef quality. However, the relationship between muscle fiber type and beef tenderness was still controversial. The mechanism and effects of muscle fiber types on postmortem beef tenderness was still under discussion.Longissimus thoracis（LT）, psoas major（PM） and semitendinosus（ST）, which are composed by different ratio of muscle fiber types, were used to study how muscle fiber types influence beef tenderness. ATPase histochemistry staining, RT-PCR, western-blotting and transmission electron microscopy technology were used to study muscle fiber characteristics, myofibril protein degradation and myofibril ultrastructure of three major muscles. i TRAQ proteomics technology was used to study differential protein expression model of three major muscles, and the aim was to study the mechanism of differences in tenderness of three major muscles. This study can provide theoretical basic for fine aging of different muscles. The paper includes four parts and the results as follows:1. Histochemistry staining and RT-PCR technologies were used to analysis muscle fiber characteristics, and the relationship between muscle fiber type composition and beef quality was studied. Results showed that the diameter order was type I < type IIA < type IIB. PM, LT and ST had the highest percentage of type I, type IIA and type IIB fibers, respectively. PM had higher p H24 h, springiness and pressing loss, and had lower Warner-Bratzler shear force（WBSF）, hardness and resilience. LT and ST had higher WBSF, hardness and resilence. ST had higher L* and h values than LT and PM（P < 0.05）. Muscle fiber diameter and area of type I, IIA and IIB fibers were positively correlated to WNSF and hardness（P < 0.05）. The percentage of type I fiber was negatively correlated to WBSF and hardness（P < 0.05）. The percentage of type IIB fiber was positively correlated to WBSF and hardness（P < 0.05）. In conclusion, to increase the percentage of type I fiber and to decrease the percentage of type IIB fiber can improve beef tenderness.2. The change regularity of postmortem aging beef quality in LT, PM and ST were studied. During the early postmortem period, PM had higher p H decline rate than LT and ST. PM had lower pressing loss than LT and ST. With the increasing of aging time, WBSF of three major muscles were decreased（P < 0.05）. Within 21 d aging, LT had higher WBSF than ST and PM, and ST had higher WBSF than PM（P < 0.05）. The tenderization rate order of three major muscles was ST > LT > PM. Muscle fiber diameter and area were positively correlated with △ WBSF（P < 0.01）. Number percentage of type I fiber was correlated to % change WBSF（r =-0.303）, and area percentage of type I fiber was correlated to △ WBSF（r =-0.474）. △ WBSF has a relationship with number percentage（r = 0.378） and area percentage（r = 0.383） of type IIB fiber. PM had lower hardness, springiness, adhesiveness and chewiness values than LT and ST. And with the increasing of aging time, hardness, springiness, adhesiveness and chewiness were decreased. ST had the highest L* and PM had the highest a* values. With the increasing of aging time, a*, b*, C and h values increased. In conclusion, muscle fiber type influences beef tenderization rate. PM, which had higher percentage of type I fiber, had lower tenderization rate. ST, which had higher percentage of type IIB fiber, had higher tenderization rate.3. Postmortem changes of glycolysis, ultrastructure, troponin T, desmin and μ-calpain in LT, PM and ST were studied. At 1 h and 6 h postmortem, PM had the highest R250 and R248, while ST had the lowest R250 and R248. With the increasing of aging time, R250 and R248 were increased, while R258 decreased. Sarcomere length of PM was higher than LT. The fracture of Z line in ST was faster than PM. The content of troponin T in PM was lower than LT and ST. The troponin T degradation bands of 38, 36, 30 and 28 k Da appeared in LT and ST muscles at 1 h postmortem, while degradation bands of 30 and 28 k Da appeared at 6 h postmortem. 50 k Da degradation band of desmin in LT appeared at 3 d postmortem and 47 k Da degradation band of desmin in PM appeared at 1 h postmortem. 50 k Da degradation band of desmin in ST appeared at 1 h postmortem, while 47 k Da degradation band appeared at 21 d postmortem. At 1 h postmortem, μ-calpain of three muscles were autolysis, and μ-calpain content in LT and ST was higher than PM. In conclusion, the degradation rate of troponin T in PM was lower than ST, while the degradation rate of desmin in PM was faster than LT and PM. And these differences were closely related with muscle fiber type and muscle specific characteristics.4. i TRAQ was used to study protein profiling of LT, PM and ST, which collected at 24 h postmortem. A total of 1123 proteins were identified. Among them, the differentially accumulated proteins number（fold change > 1.2, P < 0.05） in ST₁21-VS-LT₁13, PM₁15-VS-LT₁13 and ST₁21-VS-PM₁15 was 64, 112 and 153, respectively. The number of common differential accumulated proteins among these three groups was 11, and these proteins are troponin T（slow skeletal muscle）, myosin regulatory light chain 2, cytochrome c, heat shock protein β6 and so on. These differentially accumulated proteins were investigated by bioinformatic analysis. And the differentially accumulated proteins were applied to Gene Ontology annotation and these proteins involved in cell process, metabolic process and response to stimulus, mainly distributed in cell, organelle and membrane, and have binding, catalytic activity, enzyme regulator activity and antioxidant activity. The differentially expression proteins were assigned to 24 categories using the Clusters of Orthologous Groups of proteins（COG） database and the main functional catagories were general functional prediction only, posttranslational modification, protein turnover, chaperone, energy production and conversion, cytoskeleton and carbohydrate transport and metabolism. These differentially accumulated proteins were investigated using the KEGG database and were found to be enriched in metabolic pathway, oxidative phosphorylation, citrate cycle（TCA cycle） and cardic muscle contraction. We found that the accumulated proteins can be related to beef tenderness through TCA cycle, oxidative phosphorylation, response to stress, cytoskeleton and regulation proteins, and proteolysis.In conclusion, muscle fiber type can partly explain the different background tenderness and tenderization rate of different muscles through the difference of glycolysis, myofibrillar protein and protein profiling. Muscles, with higher percentage of type I fibers, have improved beef tenderness, but lower the tenderization rate. While muscles, with higher percentage of type IIB fibers, have higher WBSF and tenderization rate. So it is important to study tenderness difference of individual muscles from the perspective of muscle fiber type composition.